Methods Diagnosis codes for hospitalizations in 1996–2012 among previously identified population-based cohorts of patients with RA or GCA were examined for RA or GCA mentions.

Results RA or GCA mention occurred in only 55% of 2407 hospitalizations among patients with RA and 31% of 502 hospitalizations among patients with GCA. RA or GCA was mentioned more often in recent years, younger patients, and rheumatic medication users.

Conclusion Coding for RA or GCA during hospitalizations was often missed. Research using hospital diagnoses alone could be biased.